Sheave-type Continuously Variable Transmissions (CVTs) used in vehicles generally comprise an input pulley and an output pulley which are connected by a continuous belt or chain. Each pulley assembly may comprise opposed sheaves having a generally conical configuration. The continuous belt is disposed between the sheaves and frictionally engaged with each sheave. Rotation of the input pulley is imparted to the continuous belt or chain which, in turn, transfers torque to the output pulley thereby rotating the output pulley. The spacing between the sheaves of each pulley may be varied by adjusting the position of one sheave relative to the other along the axis of rotation of the pulley. This is generally accomplished by applying pressure to one of the sheaves while the other remains fixed. As the spacing between the sheaves is adjusted, the position of the continuous belt or chain is varied radially relative to the sheaves. Adjusting the spacing of the sheaves, and therefore the position of the continuous belt or chain disposed between the sheaves, adjusts the drive ratio between the input pulley and the output pulley and, as such, the transmission ratio of the vehicle.
The CVT sheaves are generally conical in shape with a relatively smooth surface which permits adjusting the continuous belt or chain relative to the sheaves as the spacing between the sheaves is adjusted. However, during high-torque duty cycles, the continuous belt or chain may slip relative to the sheaves as the high-torques imparted to the continuous belt or chain by the input pulley overcomes the frictional forces engaging the continuous belt with the surface of the sheaves. The slippage causes the continuous belt to wear and eventually break and/or malfunction. Further, wear debris from the belt may contaminate the CVT and lead to malfunctions elsewhere in the transmission. If slippage of the continuous belt can be prevented, wear would be reduced along with the amount of wear debris generated. Also, if the amount of slippage is reduced, it may be possible to reduce the pressures applied to the sheaves which engage the sheaves with the continuous belt, which, in turn, would reduce parasitic losses in the transmission. This would effectively improve the fuel economy of vehicles using this type of transmission.
Accordingly, a need exists for alternative designs for sheaves for continuously variable transmissions.